55

55 = 5 x 11.

55 is the tenth Fibonacci number.

55 is the tenth triangular number.

55 is a pyramidal number, the sum of the first five square numbers: 55 = 1² + 2² + 3² + 4² + 5².

55 is a number that cannot be written as a sum of three squares.

An emerald anniversary celebrates 55 years.

Source: Phillips, R. 2004. Numbers: Facts, Figures and Fiction. Badsey Publications.